1. Field of the Invention
The present invention generally relates to a digital signal reproducing apparatus for reproducing a digital signal recorded on an optical disc, and especially relates to the digital signal reproducing apparatus having a clock signal generating circuit for generating a clock signal which is synchronized with reproduction timing of the digital signal recorded on the optical disc.
2. Description of the Related Art
FIG. 10 is a simplified block diagram showing a construction of a CD (Compact Disc) player, which is one example of the digital signal reproducing apparatus.
In FIG. 10, CD 30, which is one example of an optical disc, is rotated in rotation velocity according to a predetermined linear velocity by a spindle motor 2. The spindle motor 2 is driven on the basis of a rotation control signal supplied from a spindle servo circuit 9. A pickup 1 irradiates a light beam to the CD 30, and accepts a reflected light which is reflected in the CD 30 and is interfered in a row of pits formed on the CD 30. Further, the pickup 1 is generates a reading signal corresponding to quantity of acceptance of the reflected light. A head amplifier 4 amplifies the generated reading signal with a suitable amplification factor, and supplies this amplified reading signal to an equalizer 5, which is one example of a waveform equalizing circuit.
The equalizer 5 has a frequency characteristic to emphasize a predetermined high frequency band in the amplified reading signal. Further, the equalizer 5 compensates a waveform of the amplified reading signal by this frequency characteristic, and supplies this compensated reading signal to comparator 6. Namely, the equalizer 5 forcibly makes the amplitude of the high frequency component in the reading signal higher in order to compensate a lack (decrease) of the amplitude the high frequency component in this signal.
The comparator 6 compares the level of the reading signal compensated its waveform by the equalizer 5 with a predetermined reference voltage, and generates a binary signal corresponding to the result of this comparison.
A clock generation circuit 7 is constructed by the so called PLL (Phase Locked Loop) circuit having: a phase comparison circuit 701 for comparing the phase of the binary signal supplied from the comparator 6 with that of a sample clock signal supplied from a VCO (Voltage Control Oscillator) 703, which is one example of a voltage control oscillator, and for outputting a phase error signal; an LPF (Low Pass Filter) 702 for restricting a frequency band of the phase error signal; and the VCO 703 for generating a clock signal according to the phase error signal outputted from the LPF 702. This clock generation circuit 7 generates the sample clock signal whose phase is synchronized with that of the edge of the binary signal.
A sampling circuit 8 samples the above mentioned binary signal at the timing corresponding to the sample clock signal, and sequentially outputs this sampled data as a reproduction digital signal.
On the other hand, this CD player further has a position detecting circuit 10 and a frequency detecting circuit 11. The position detecting circuit 10, for example, optically or mechanically reads a scale (graduation which indicates the radius position of the disc) disposed on a slider base on which a slider to move the pickup 1 in the radius direction of the disc is mounted, and outputs a position detecting signal to be used for recognizing the position of pickup 1 in the radius direction of the disc. The frequency detecting circuit 11 detects the rotation velocity (rotation frequency) of the spindle motor 2 which gives the rotation to the CD 30, and outputs a frequency detecting signal. Further, each of the detection signals outputted the position detecting circuit 10 and the frequency detecting circuit 11 is respectively supplied to a CPU 12.
The CPU 12 controls the access operation to move the pickup 1 at the determined reproduction radius position on the basis of the supplied the position detecting signal, and gives an instruction to the spindle servo circuit 9 in order to set the rotation velocity of the CD 30 at the reproduction radius position. Here, on the CD 30, information are recorded by CLV (Constant Linear Velocity). Hereinbelow, it is referred to as "CLV disc".
As mentioned above, in this CD player, firstly, the pickup 1 is moved at the determined reproduction radius position, and the spindle motor 2 i.e., CD 30 is rotated in the predetermined rotation velocity on the basis of the constant linear velocity. Then, the reading signal is read from the CD 30, and then, the binary signal is generated on the basis of the reading signal, and further, the sample clock signal synchronized with the edge of the binary signal is generated, and the reproduction digital signal is generated by sampling the binary signal at the timing corresponding to the sample clock signal, thereafter, this reproduction digital signal is outputted.
Here, when the reproduction digital signal is generated on the basis of the binary signal, the frequency of the sample clock signal must be sufficiently stable or be fixed. Therefore, in the above mentioned CD player, the reproduction digital signal is generated in the condition that the CD 30 i.e., the spindle motor 2 is sufficiently rotating in accordance with the predetermined rotation velocity on the basis of the constant linear velocity.
However, the weight of the disc affects the spindle motor 2 as load, for example, when the rotation velocity of the disc is changed. Therefore, response speed of the spindle motor 2 is generally bad (slow) because of inertia force of the load. Thus, in the CD player for reproducing the CLV disc, the time period when the pickup 1 is moved to the determined reproduction radius position from a different position and the operation to read the record information from the disc is started, i.e., the access time period, is long. Namely, the time period when the rotation velocity of the spindle motor 2 changes to and reaches the predetermined rotation velocity at the reproduction radius position is long. On the other hand, if the torque of the spindle motor 2 is increased, its response speed is fast. However, if the torque is increased, large electric current need to be applied to the spindle motor 2, the spindle servo circuit 9, etc, so that the temperature in the apparatus (CD player) rise because of heat generated by the applied electric current. Therefore, a cooling device, a cooling system, etc need to be added to the CD player, so that the size of the apparatus (CD player) becomes large. Especially, in case of a reproducing apparatus for reproducing a CD-ROM on which computer software is recorded, which is another example of the digital signal reproducing apparatus, since this reproducing apparatus is generally disposed in the housing of computer, the size of this reproducing apparatus is restricted. Therefore, it is undesirable that the size of this apparatus becomes large.
In order to solve the aforementioned problem and undesirability, it is to be desired that the operation to read the record information from the disc can be started when the access operation is finished, i.e., when the operation that the pickup 1 is moved as far as the determined reproduction radius position is finished. That is to say, it is to be desired that the operation to read the record information from the disc can be started without waiting for the time period when the rotation velocity of the spindle motor 2 reaches the predetermined rotation velocity at the reproduction radius position. In order to satisfy this desire, it is require that even if the rotation velocity of the spindle motor 2 i. e., CD 30 dose not become the predetermined rotation velocity by the constant linear velocity, the clock signal generating circuit 7 generates the sample clock signal whose frequency and phase are synchronized with those of the edge of the binary signal on the basis of the reading signal read from the CD 30 at the reproduction radius position in order to reproduce data (in order to output this reproduction digital signal).